1. Field of the Invention
The present invention relates to a cleanerless image forming method and apparatus which is applied, for example, to a printer of an electrophotographic system.
The present invention further relates to an image forming apparatus of the type in which light is exposed to an image carrier to form an electrostatic latent image, and said image is developed by a colored powder to transfer it to a recording medium, and in particular, to an image forming apparatus having a scrotron type transfer means.
2. Prior Art
FIG. 14 is a schematic structural view showing parts around an image carrier of a conventional image forming apparatus of the type as described above. The apparatus comprises a photosensitive drum 1, a charger 2, an exposure means 3, a developer 4, a bias power source 5 of the developer, a recording paper 8, a transfer unit 15, and a cleaner 16. Ordinarily, as the image carrier, a drum-like photosensitive body is used as in this example. The photosensitive drum 1 rotates in a direction as indicated by the arrow.
The image forming process is started by uniformly charging the photosensitive drum 1 by the charger 2, and then an electrostatic latent image is formed by the exposure means 3. The exposure is effected by the slit exposure using an optical system, or the exposure is effected by a laser beam modulated by an image signal. The latent image is developed by toner as a colored powder using the developer 4, and transferred to the recording paper 8 by the transfer unit 15 to effect image formation.
The toner remained on the photosensitive drum after transfer is cleaned by the cleaner 16. As the cleaner 16, a blade 16a is used to scrape off the residual toner and is delivered into an external toner recovery box not shown by a carrying screw 16b.
The cleaner of the scrape-off type using the blade requires a space for toner recovery box. It is necessary to monitor if the toner recovery box is filled or not. There exists a further problem in that the surface of the photosensitive drum 1 is liable to be scrached by the blade 16a to shorten the life of the photosensitive drum.
A cleaning system without using a scrape-off blade has also been proposed. In an image forming apparatus described in Japanese Patent Publication Unexamined No. 64-20587, there is used a memory removing member. FIG. 15 is a schematic structural view showing parts around the image carrier. Same parts as those shown in FIG. 14 are indicated by the same reference numerals, and description thereof will be omitted. Reference numeral 6 denotes a scorotron type transfer unit; 7 denotes a high voltage power source; 9 denotes a varister; 14 denotes a memory removing member; and 17 denotes a high voltage power source. In this image forming apparatus, it can be used for external output devices such as an electronic computer, a word processor, and so on.
In recording, the photosensitive drum 1 is rotated according to a print start signal, and the charge is made by the charger 2. The charging potential is V.sub.o shown in FIG. 17, for example, -700 V. In the exposure means 3, a laser beam modulated by a dot image data from the electronic computer, the word processor, etc. scans and exposes the photosensitive drum 1 to form an electrostatic latent image.
FIG. 16(A) is an explanatory view of the charging state in the case where toner remained on the photosensitive drum 1 is present. Since the photosensitive drum 1 is charged to the charging potential V.sub.o, residual toners a and b are also charged to the same potential V.sub.o. The photosensitive body under the residual toners a and b are also charged to the same potential V.sub.o.
FIG. 16(B) is an explanatory view of the exposure state. The exposed portion forms an electromagnetic latent image which will be the exposure potential V.sub.c shown in FIG. 17, and therefore, the residual toner b at that portion will also be the exposure potential V.sub.c. In the case where the residual toner in the exposure portion is scattered in a small area, for example, one by one, the photosensitive body under the residual toner is also photo-sensitized. In the case where it is gathered in a large area, the photosensitive body thereunder is short in exposure. However, in the case where the exposure-short portion is embedded by toner adhered to a peripheral exposed portion, gathering of residual toner poses no problem. Accordingly, as will be described later, if the residual toner is scatterred to the extent such that the exposure is not affected thereby by the memory removing member, the residual toner can be substantially disregarded.
The developer 4 develops a latent image at the developing potential V.sub.b1, for example, -450 V. As shown in FIG. 16(C), toner in the developer is developed to the exposed portion and adherred thereto. The residual toner b in the exposed portion remains stayed but poses no problem. Residual toner a in a non-exposed portion is returned to the developer 4, and the toner can be cleaned. A cleaning potential is expressed by a difference between the charging potential V.sub.o and the developing potential V.sub.b1 below: EQU V.sub.CL =V.sub.b1 -V.sub.o
The transfer is effected by a scorotron type charger. By use of the scorotron type, a voltage in excess of 5 kV can be applied to a corona wire 6a by a high voltage power source 7 to stabilize the discharge. A varister 9 is connected to a grid 6b, and a constant voltage, for example, 560 V, is generated by a part of corona current. Accordingly, a grid potential is 560 V. Toner developed by a transfer potential is transferred to the recording paper 8, the transferred toner is fixed by a fixing unit not shown, and the recording paper is discharged. The photosensitive drum will be at a potential V.sub.r, for example, 100 V, after transfer. The non-exposed portion will be 0 to 80 V.
A memory removing potential V.sub.b2 is applied to the photosensitive drum 1 by the memory removing member 14 connected to the high voltage power source 17. The appropriate memory removing potential V.sub.b2 is 100 to 700 V. As the memory removing member 14, a brush is used. The memory removing member 14 has its function that the residual toner is once electrostatically attracted, after which it is naturally discharged to the photosensitive drum 1, and the adherring position on the photosensitive drum 1 is changed whereas as a result, a pattern of the residual toner is diffused.
By the aforementioned function, the residual toner does not obstruct the succeeding exposure, and the residual toner in the non-exposure portion is recovered into the developer simultaneously with the development. This means that cleaning has been done.
The image forming apparatus using the aforementioned cleanerless method is very excellent in that a cleaner is not used. However, there is an inconvenience in that when a power source is opened due to an occurrence of trouble such as jams or when defective transfer of a toner image onto a paper for other causes, un-transferred toner more than that needed are adherred to and staryed on the memory removing member. This results in drawbacks that the memory removing effect (the scattering effect of the residual toner image) is insufficient to render the succeeding image forming operation inconvenient or that toner overflows from the memory removing member to stain paper running thereunder or stain marginal portions.
Furthermore, in the aforementioned image forming process, paper powders is often deposited on the surface of the photosensitive drum. The paper powder generated include those generated when paper are cut into fixed forms, or those generated by contact of paper with rollers, guides, etc. when the paper runs within the apparatus. Further, paper powders is materially generated particularly when reproduced papers is used, resulting in that a large amount of paper powders is adherred to the surface of the photosensitive drum. Defective exposure, defective development, defective transfer, etc. are brought forth at portions where paper powders is adherred unless the powders is adherred, constituting an obstacle to form an image.
Accordingly, it is very important to provide the step of recovering paper powder together with untransferred toner by means of the aforementioned cleaner. However, in the cleanerless image forming apparatus without using a cleaner, paper powder together with untransferred toner are recovered into the developer, resulting in the following inconvenience. That is, the function peculiar to the developer is to make toner within a toner hopper thin by means of a developing roller and a doctor blade to supply a fixed amount of toner to an electrostatic latent image on the surface of the photosensitive drum. However, in the case where paper powder is mixed into the toner hopper, the paper powder is carried along with toner by the developing roller to a position of the doctor blade, at which position the paper powder is clogged.
The grain size of toner is approximately 10-20 .mu.m, whereas the grain size of paper powder is 0.3 mm or more, and therefore, the paper powder cannot pass through a doctor gap (an isolated distance between the doctor blade and the developing roller) and is dammed up by the doctor blade. In the case where such an inconvenience occurs, at a location where paper powder at the position of the doctor blade is clogged, a toner supply-disable state (not-developed state) with respect to the electrostatic latent image on the photosensitive drum results. This causes an image to be formed having with portions whited out (i.e., without toner).
In view of the foregoing, it is desirable to remove paper powder by some method before it is supplied to a transfer unit. However, it is impossible to completely remove paper powder from paper in consideration of nature peculiar to paper. Furthermore, it is necessary to bring paper into contact with the photosensitive drum once without fail in the transfer step, and therefore, adherence of some paper powder to the surface of the photosensitive drum cannot be avoided. Accordingly, in the conventional cleanerless image forming apparatus in which positive removable of paper powder by a cleaner is not effected, it is extremely difficult to completely remove paper powder on the photosensitive drum.
Moreover, when such a residual toner recovery system is employed, the charge of the photosensitive body caused by the transfer unit 6 poses a problem. A potential of the photosensitive body having passed through the transfer unit 6 is affected by the presence or absence of the recording paper. Assume now that in case of presence of the recording paper, a potential V.sub.T0 after transfer is 100 V, a potential V.sub.T1 after transfer in case of absence of the recording paper is approximately 400 V. When the photosensitive body is charged to approximately 400 V, which is V.sub.T1, the toner adherred to the memory removing member 14 becomes developed on the photosensitive body. The toner in this phenomenon will not be a scatterred pattern and therefore constitutes an obstacle in the succeeding exposure stage.
Accordingly, the cleaning system described in the aforementioned patent publication, a voltage applied to the corona wire 6a of the transfer unit 6 is turned on only when the recording paper 8 is passing through under the transfer unit 6, and exposed portions of the photosensitive body before and behind the recording paper 8 are not positively charged.
However, it is difficult to accurately synchronize a carrying position of the recording paper 8 with the turning-on of the high voltage power source 7. Other problems include rise time and fall time when the power source is turned on and off, making it difficult to avoid the charging of the exposed portions of the photosensitive body.
In addition, there is a problem in that a high voltage power source 17 is required for the memory removing member 14 as compared with the case of using a cleaner of the conventional scrape-off system using a blade.
While a description has been made by using a negative charge OPC as a photosensitive drum, it is to be noted that the same is true for a positive-charge OPC.